For a relatively small country, New Zealand has a long coastline, and jurisdiction over a very large area of sea. There is approximately 19,000 kilometres of coastline for the mainland, with 20,500 kilometres when the offshore islands and the Chatham Islands are included. This coastline borders a marine area extending over 5.8 million square kilometres (see Figure 1.1). New Zealand’s Exclusive Economic Zone (EEZ) is estimated at 4,000,000 square kilometres and its Extended Continental Shelf (ECS) estimated at 1,700,000 square kilometres. The EEZ is 15 times larger than the land area, and the EEZ with the continental shelf included are 21 times land area.
But the marine environment is considerably larger than this two-dimensional figure indicates. It also has a substantial third dimension, extending down from the surface of the sea, to the seabed. Although the sea is relatively shallow in coastal areas, where the land abuts the continental shelf, seventy-five per cent of New Zealand’s marine area has depths of more than one kilometre. The deepest place is within the Kermadec Trench, some ten kilometres from the surface.
New Zealand’s seabed has a very diverse topography. This is because the country sits astride two colliding tectonic plates. The movement of the plates has resulted in arcs of volcanic activity and the uplift and depression of large areas of coastal land and seabed.
Many coastal features have been formed from the remnants of volcanic activity including cones, sunken craters and lahars. New Zealand’s seabed features chains of underwater volcanoes and deep ocean trenches and ridges. The mountains which are entirely under the sea are called ‘seamounts’
Closer to land, the New Zealand coastline is punctuated by many estuaries and harbours. These shallow sheltered areas were formed after the last ice age, when the rising seas flooded river valleys and other low-lying areas. They have continued to infill with sediment washed off the land.
Rocky reefs have formed around the more exposed parts of the coast, with soft rock being eroded away by waves to form bays, and harder rock remaining as headlands and reefs. Sand spits and barrier islands are also common around the coast, forming across the mouths of harbours and bays, where rivers have dropped their sediment load.
Movement of water
In terms of the sea itself, New Zealand’s marine area is influenced by two major bodies of oceanic water; subtropical surface water which travels from the central Pacific Ocean via the eastern coast of Australia; and subantarctic surface water which originates from the Southern Ocean.
Both of these water bodies move in an easterly direction and they meet along a subtropical front which bisects the southern part of New Zealand’s marine area. This oceanic meeting point is located along the same latitude as Fiordland on the west coast and along the Chatham Rise to the east (as shown in Figure 1.2). It is a very productive area for marine life, particularly over the Chatham Rise. More localised currents move the water around the New Zealand coast.
Upwelling is a process produced by the movement of water and is important for nutrient cycling. This process is discussed further in the section on marine productivity.
Last updated at 2:11PM on February 25, 2015